Underline Theorem's 'Theorem' part, not the title

Question

I would like to have two different styles of theorems in my LaTeX document to differentiate between original and unoriginal work. For one I would like the 'Theorem' underlined and the other I would like without extra text decoration (as normal).

I use the amsthm package for theorems usually.

---

For example I would like two different theorems that look like the following would.

\textbf{Theorem 2.2.} \textit{blah blah blah}

\underline{\textbf{Theorem 2.3.}} \textit{blah blah blah}

Answer 1

Like this?

\documentclass{article}
\usepackage{amsmath}
\usepackage[utf8]{inputenc}
\usepackage[english]{babel}
\usepackage[amsmath,thmmarks]{ntheorem}

\makeatletter 
 \newtheoremstyle{myu}%
  {\item[\hskip\labelsep \ \bf\underline{##1 \theorem@headerfont ##2.}]}%
\makeatother
\makeatletter 
 \newtheoremstyle{myn}%
  {\item[\hskip\labelsep \ \bf ##1 \theorem@headerfont ##2.]}%
\makeatother


\theoremstyle{myn}
\newtheorem{theoremn}{Theorem} %<-- Normal Theorem Definition
\theoremstyle{myu}
\newtheorem{theoremu}[theoremn]{Theorem}%<-- Underlined Theorem Definition

\begin{document}
\section{Introduction}
Theorems can easily be defined
\begin{theoremn}
Let $f$ be a function whose derivative exists in every point, then $f$ 
is a continuous function.
\end{theoremn}

\begin{theoremu}
Let $f$ be a function whose derivative exists in every point, then $f$ 
is a continuous function.
\end{theoremu}

\begin{theoremn}
Let $f$ be a function whose derivative exists in every point, then $f$ 
is a continuous function.
\end{theoremn}

\begin{theoremu}
Let $f$ be a function whose derivative exists in every point, then $f$ 
is a continuous function.
%\noindent\underline{\makebox[3mm][l]{Jane Doe}}
\end{theoremu}

\begin{theoremu}
Let $f$ be a function whose derivative exists in every point, then $f$ 
is a continuous function.
\end{theoremu}
\end{document}